Crystalline 1α-hydroxyvitamin D2 and method of purification thereof

ABSTRACT

A method of purifying 1α-hydroxyvitamin D 2  to obtain 1α-hydroxyvitamin D 2  in crystalline form. The method includes the steps of boiling a solvent selected from the group consisting of ethyl formate, ethyl acetate and a 2-propanol-hexane mixture under inert atmosphere, dissolving a product containing 1α-hydroxyvitamin D 2  to be purified in the. solvent, cooling the solvent and dissolved product below ambient temperature for a sufficient amount of time to form a precipitate of 1α-hydroxyvitamin D 2  crystals, and recovering the 1α-hydroxyvitamin D 2  crystals. Optionally, petroleum ether may be added to the solvent after dissolving the product to be purified in the solvent.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support awarded by the National Institutes of Health (NIH), Grant #DK-14881. The United States Government has certain rights in this invention.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to purification of organic compounds, and more particularly to the purification of 1α-hydroxyvitamin D₂ (1αOH-D₂) by preparing it in crystalline form.

Purification of organic compounds, especially those designated for pharmaceutical use, is of considerable importance for chemists synthesizing such compounds. Preparation of the compound usually requires many synthetic steps and, therefore, the final product can be contaminated not only with side-products derived from the last synthetic step of the procedure but also with compounds that were formed in previous steps. Even chromatographic purification, which is a very efficient but relatively time-consuming process, does not usually provide compounds which are sufficiently pure to be used as drugs.

Depending on the method used to synthesize 1α-hydroxyvitamin D compounds, different minor undesirable compounds can accompany the final product. Thus, for example, if direct C-1 hydroxylation of 5,6-trans geometric isomer of vitamin D is performed, followed by SeO₂/NMO oxidation and photochemical irradiation [see Andrews et al., J. Org. Chem. 51, 1635 (1986); Calverley et al., Tetrahedron 43, 4609 (1987); Choudry et al., J. Org. Chem. 58, 1496 (1993)], the final 1α-hydroxyvitamin D product can be contaminated with 1β-hydroxy- as well as 5,6-trans isomers. If the method consists of C-1 allylic oxidation of the 4-phenyl-1,2,4-triazoline-3,5-dione adduct of the previtamin D compound, followed by cycloreversion of the modified adduct under basic conditions [Nevinckx et al., Tetrahedron 47, 9419 (1991); Vanmaele et al., Tetrahedron 41, 141 (1985) and 40, 1179 (1991); Vanmaele et al., Tetrahedron Lett. 23, 995 (1982)], one can expect that the desired 1α-hydroxyvitamin can be contaminated with the previtamin 5(10),6,8-triene and 1β-hydroxy isomer. One of the most useful C-1 hydroxylation methods, of very broad scope and numerous applications, is the experimentally simple procedure elaborated by Paaren et al. [see J Org. Chem. 45, 3253 (1980) and Proc. Natl. Acad. Sci. U.S.A. 75, 2080 (1978)]. This method consists of allylic oxidation of 3,5-cyclovitamin D derivatives, readily obtained from the buffered solvolysis of vitamin D tosylates, with SeO₂/t-BuOOH and subsequent acid-catalyzed cycloreversion to the desired 1α-hydroxy compounds. Taking into account this synthetic path it is reasonable to assume that the final product can be contaminated with 1β-hydroxy epimer, 5,6-trans isomer and the previtamin D form. 1α-hydroxyvitamin D₄ is another undesirable contaminant found in 1α-hydroxyvitamin D₂ synthesized from vitamin D₂ or from ergosterol. 1α-hydroxyvitamin D₄ results from C-1 oxidation of vitamin D₄, which in turn is derived from contamination of the commercial ergosterol material. Typically, the final product may contain up to about 1.5% by weight 1α-hydroxyvitamin D₄. Thus, a purification technique that would eliminate or substantially reduce the amount of 1α-hydroxyvitamin D₄ in the final product to less than about 01.-0.2% would be highly desirable.

The vitamin D conjugated triene system is not only heat- and light-sensitive but it is also prone to oxidation, leading to the complex mixture of very polar compounds. Oxidation usually happens when a vitamin D compound has been stored for a prolonged time. Other types of processes that can lead to a partial decomposition of vitamin D compounds consist of the some water-elimination reactions; their driving force is allylic (1α-) and homoallylic (3β-) position of the hydroxy groups. The presence of such above-mentioned oxidation and elimination products can be easily detected by thin-layer chromatography. Thus, for example, using precoated aluminum silica sheets [with UV indicator; from EM Science (Cherry Hill, N.J.)] and solvent system hexane-ethyl acetate (4:6), the spot of 1α-OH-D₂(R_(f) 0.27) and its elimination products (R_(f)'s ca. 0.7-0.9) are visible in ultraviolet light. Also, after spraying with sulfuric acid and heating, an additional spot can be visualized (R_(f) 0), derived from oxidation products.

Usually, all 1α-hydroxylation procedures require at least one chromatographic purification. However, even chromatographically purified 1α-hydroxyvitamin D₂, although showing consistent spectroscopic data, suggesting its homogeneity, does not meet the purity criteria required for therapeutic agents that can be orally, parenterally or transdermally administered. Therefore, it was evident that a suitable method of purification of 1α-hydroxyvitamin D₂ is required.

Since it is well known that the simplest procedure that can be used for compound purification is a crystallization process, it was decided to investigate purification of 1α-OH-D₂ by means of crystallization. The solvent plays a crucial role in the crystallization process, and is typically an individual liquid substance or a suitable mixture of different liquids. For crystallizing 1α-hydroxyvitamin D₂, the most appropriate solvent and/or solvent system is characterized by the following factors:

(1) low toxicity;

(2) low boiling point;

(3) significant dependence of solubility properties with regard to temperature (condition necessary for providing satisfactory crystallization yield); and

(4) relatively low cost.

It was found that highly apolar solvents (e.g. hydrocarbons) were not suitable due to the low solubility of 1α-OH-D₂ in them. Quite the reverse situation occurred in highly polar solvent media (e.g. alcohols), in which 1αOH-D₂ showed too high solubility. Therefore, it was concluded that for the successful crystallization of 1α-OH-D₂, a solvent of medium polarity is required or, alternatively, a solvent mixture consisting of two (or more) solvents differing considerably in polarity. Interestingly, hexane, so frequently used for crystallization purposes with co-solvents like acetone, ethyl acetate or diethyl ether, was found less suitable for crystallization of 1α-OH-D₂. Unusually low yields of crystallization were obtained when hexane-containing solvent systems were used. However, it was discovered that replacement of the hexane in such solvent mixtures with petroleum ether increased significantly the yield of crystals. After numerous experiments it was found that an individual solvent, namely ethyl formate, was most useful for the crystallization of 1α-OH-D₂. In addition, binary and ternary solvent systems namely: ethyl acetate-petroleum ether and 2-propanol-hexane-petroleum ether, respectively, also performed well. These solvents are all characterized by low toxicity, and they are very easy to remove by evaporation or other well known methods. In all cases the crystallization process occurred easily and efficiently; and the precipitated crystals were sufficiently large to assure their recovery by filtration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1 h are graphs of ¹H NMR spectra (CDCl₃, 500 MHz) of the crystals of 1α-hydroxyvitamin D₂ resulted after two crystallizations using the following solvent system: HCOOEt (FIGS. 1a and 1 b), AcOEt-petroleum ether (FIGS. 1c and 1 d) and iPrOH-hexane-petroleum ether (FIGS. 1e and 1 f) as well as the spectrum of the solid 1α-hydroxyvitamin D₂ material before crystallization (FIGS. 1g and 1 h);

FIGS. 2a-2 d are HPLC (10 mm×25 cm Zorbax-Sil column, 15% 2-propanol in hexane, 4 mL/min; UV detection at 260 nm) profiles of the solid 1α-hydroxyvitamin D₂ material before crystallization (FIG. 2a) and the crystals resulted after two crystallizations using the following solvent system: HCOOEt (FIG. 2b), AcOEt-petroleum ether (FIG. 2c) and iPrOH-hexane-petroleum ether (FIG. 2d). In the region indicated by the asterisk (ca. 36 mL) sensitivity was decreased 20 times.

FIGS. 3a-3 i are HPLC (10 mm×25 cm Zorbax-Sil column, 15% 2-propanol in hexane, 4 mL/min; UV detection at 260 nm) profiles of the crystals of 1α-hydroxyvitamin D₂ resulted after single crystallization using the following solvent system: HCOOEt (FIG. 3a), AcOEt-petroleum ether (FIG. 3d) and iPrOH-hexane-petroleum ether (FIG. 3g); the HPLC profiles of mother liquors after single crystallization using the following solvent systems: HCOOEt (FIG. 3b), AcOEt-petroleum ether (FIG. 3e) and iPrOH-hexane-petroleum ether (FIG. 3h); and the HPLC profiles of mother liquors after two crystallizations using the following solvent system: HCOOEt (FIG. 3c), AcOEt-petroleum ether (FIG. 3f) and iPrOH-hexane-petroleum ether (FIG. 3i). Region with decreased sensitivity (ca. 36 mL) is indicated by asterisk.

FIGS. 4a-4 f are Microscope-magnified images of the crystals of 1α-hydroxyvitamin D₂ resulted after two crystallizations using the following solvent system: HCOOEt (FIG. 4a-40×, FIG. 4b-100×), AcOEt-petroleum ether (FIG. 4c-100×, FIG. 4d-400×) and iPrOH-hexane-petroleum ether (FIG. 4e-100×, FIG. 4f-400×).

FIGS. 5a-5 d are HPLC (4.6 mm×25 cm Zorbax-Eclipse XDB-C18 column, 7% water in methanol, 1.5 mL/min; UV detection at 260 nm) profiles of the solid 1α-hydroxyvitamin D₂ material before crystallization (FIG. 5a) and the crystals resulted after two crystallizations from: HCOOEt (FIG. 5b), AcOEt-petroleum ether (FIG. 5c) and iPrOH-hexane-petroleum ether (FIG. 5d). In the region indicated by asterisk (ca. 15 min) sensitivity was decreased 20 times.

FIGS. 6a and 6 b are illustrations of the three dimensional structure of 1α-hydroxyvitamin D₂ as defined by the atomic positional parameters discovered and set forth herein.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a valuable method of purification of 1α-hydroxyvitamin D₂, a pharmacologically important compound, characterized by the formula shown below:

The purification technique involves obtaining the 1α-hydroxyvitamin D₂ product in crystalline form by utilizing a crystallization procedure wherein the 1α-hydroxyvitamin D₂ material to be purified is dissolved using as the solvent or solvent system one of the following:

(1) a single solvent system, namely, ethyl formate;

(2) a binary solvent system, namely, ethyl acetate and petroleum ether; or

(3) a ternary solvent system, namely, 2-propanol in combination with hexane and petroleum ether.

Thereafter, the solvent or solvent system can be removed by evaporation, with or without vacuum, or other means as is well known. The technique can be used to purify a wide range of final products containing 1α-hydroxyvitamin D₂ obtained from any known synthesis thereof, and in varying concentrations, i.e. from microgram amounts to kilogram amounts. As is well known to those skilled in this art, the amount of solvent utilized should be minimized and/or adjusted according to the amount of 1α-hydroxyvitamin D₂ to be purified.

The usefulness and advantages of the present crystallization procedures is shown in the following specific Examples. Solid 1α-hydroxyvitamin D₂ product, obtained by Paaren's, supra, method, purified by flash chromatography on silica, and stored for few months in refrigerator, was used as a suitable starting material. Although this material still showed reasonably good 500 MHz ¹H NMR spectrum (FIGS. 1g, 1 h), concomitant compounds were detected by straight- and reverse-phase HPLC (FIGS. 2a and 5 a, respectively) and, moreover, the presence of some oxidation products was confirmed by TLC (presence of a spot at R_(f) 0). After recrystallization from the solvents listed above, the precipitated material was observed under microscope to confirm its crystalline form (FIGS. 4a-4 f). Additionally, in the case of crystals precipitated from methyl formate, X-ray diffraction analysis was performed. The corresponding crops of crystals were then carefully analyzed and their significantly improved purity was confirmed by straight-phase HPLC (FIGS. 2b, 2 c, 2 d), reverse-phase HPLC (FIGS. 5b, 5 c, 5 d), TLC and 500 MHz ¹H NMR (FIGS. 1a-1 f). Yields of crystallizations were high and the obtained crystals showed a relatively sharp melting point.

As it can be seen from FIGS. 2b and 2 c, HPLC profiles of 1α-hydroxyvitamin D₂ obtained after two crystallizations from ester-containing solvents indicate presence of a small amount of less polar impurities (peaks at R_(v) ca. 18 mL) which most likely originate from the corresponding 1α-OH-D₂ formate(s) and acetate(s) formed in minimal quantities in the equilibrium processes:

HCOOEt(CH3COOEt)+ROH⇄HCOOR(CH3COOR)+EtOH

However, the small amount of such esterificated compounds (less than 0.4%) presents no problem for pharmacological application of the crystalline vitamin D₂ compound due to the well-known fact that vitamin D esters undergo hydrolysis in living organisms.

Also, the corresponding reverse -phase HPLC profiles of the recrystallized 1α-hydroxyvitamin D₂, shown in FIGS. 5b, 5 c and 5 d, clearly indicate a considerable improvement in the compound's purity. The important observation consists of the significantly diminished proportion of the concomitant 1α-hydroxyvitamin D₄ (peak of retention time ca. 18 mL) in the recrystallized compound. The content of this impurity has decreased more than 4 times (4.1-4.3) in respect to its value in the starting 1α-hydroxyvitamin D₂ product and does not exceed 0.2%.

The described crystallization processes of the synthetic 1α-hydroxyvitamin D₂ product represents a valuable purification method, which can remove not only some side products derived from the synthetic path, but, moreover, concomitant 1α-hydroxyvitamin D₄. Such impurity is the result of the contamination of natural ergosterol with its 22,23-dihydro analog and, consequently, vitamin D₄ is present in different proportions in the commercially available vitamin D₂. Column chromatography and straight-phase HPLC separation of 1α-hydroxyvitamin D₄ (formed after 1α-hydroxylation process) from 1α-hydroxyvitamin D₂ is practically impossible due to their similar chromatographical properties and larger-scale separation is also difficult by reverse-phase HPLC.

CRYSTALLIZATION OF 1α-HYDROXYVITAMIN D₂ EXAMPLE 1

Crystallization from ethyl formate

(a) 1α-Hydroxyvitamin D₂ product (50 mg) to be purified was dissolved in boiling ethyl formate (1.2 mL, Aldrich) under argon atmosphere, left at room temperature (68° F.) for a few hours (1-3 hrs) and then in a refrigerator (35-45° F.) overnight (8-12 hrs). The precipitated crystals were filtered off, washed with a small volume of a cold (0° C.) ethyl formate and dried. The yield of crystalline material was 38 mg (76%). HPLC profiles of crystals and mother liquor are shown in FIGS. 3a, 3 b.

(b) These crystals of 1α-hydroxyvitamin D₂ (26.8 mg) were recrystallized with ethyl formate (0.5 mL) as described in Example 1(a) and the precipitated crystals (20 mg, 78%), m.p. 153-155° C., were observed under a microscope (FIGS. 4a, 4 b) and analyzed by straight-phase HPLC (crystals: FIG. 2b; mother liquors: FIG. 3c), reverse-phase HPLC (FIG. 5b), and ¹H NMR (FIGS. 1a, 1 b).

EXAMPLE 2

Crystallization from binary solvent system: ethyl acetate-petroleum ether

(a) 1α-Hydroxyvitamin D₂ product (50 mg) to be purified was dissolved in boiling ethyl acetate (0.5 mL, Burdick&Jackson) under argon atmosphere and petroleum ether (1.5 mL, b.p. 35-60° C.; Aldrich) was added. The solution was left at room temperature (68° F.) for a few hours (1-3 hrs) and then in a refrigerator (35-45° F.) overnight (8-12 hrs). The precipitated crystals were filtered off, washed with a small volume of petroleum ether and dried. The yield of crystalline material was 32.5 mg (65%). HPLC profiles of crystals and mother liquor are shown in FIGS. 3d, 3 e.

(b) These crystals of 1α-hydroxyvitamin D₂ (24.8 mg) were recrystallized with ethyl acetate (0.23 mL) and petroleum ether (0.69 mL) as described in Example 2(a) and the precipitated crystals (17 mg, 69%), m.p. 149.5-152.5° C., were observed under a microscope (FIGS. 4c, 4 d) and analyzed by straight-phase HPLC (crystals: FIG. 2c; mother liquors: FIG. 3f), reverse-phase HPLC (FIG. 5c), and ¹H NMR (FIGS. 1c, 1 d).

EXAMPLE 3

Crystallization from ternary solvent system: 2-propanol-hexane-petroleum ether

(a) 1α-Hydroxyvitamin D₂ product (50 mg) to be purified was dissolved in boiling 2-propanol-hexane mixture (15:85; 0.6 mL; Burdick&Jackson) under argon atmosphere and petroleum ether (1.7 mL, b.p. 35-60° C.; Aldrich) was added. The solution was left at room temperature (68° F.) for a few hours (1-3 hrs) and then in a refrigerator (35-45° F.) overnight (8-12 hrs). The precipitated crystals were filtered off, washed with a small volume of petroleum ether and dried. The yield of crystalline material was 34.5 mg (69%). HPLC profiles of crystals and mother liquor are shown in FIGS. 3g, 3 h.

(b) These crystals of 1α-hydroxyvitamin D₂ (23.6 mg) were recrystallized with 2-propanol-hexane mixture (15:85; 0.15 mL) and petroleum ether (0.4 mL) as described in Example 3(a) and the precipitated crystals (15.6 mg, 66%), m.p. 154-156° C., were observed under a microscope (FIGS. 4e, 4 f) and analyzed by straight-phase HPLC (crystals: FIG. 2d; mother liquors: FIG. 3i), reverse-phase HPLC (FIG. 5d), and ¹H NMR (FIGS. 1a, 1 f).

EXAMPLE 4

Experimental

A colorless prism-shaped crystal of dimensions 0.52×0.44×0.38 mm was selected and designated as 98247 for structural analysis. Intensity data for this compound were collected using a Bruker SMART ccd area detector; (a) Data Collection: SMART Software Reference Manual (1994). Bruker-AXS, 6300 Enterprise Dr., Madison, Wis. 53719-1173, USA; (b) Data Reduction: SAINT Software Reference Manual (1995). Bruker-AXS, 6300 Enterprise Dr., Madison, Wis. 53719-1173, USA; mounted on a Bruker P4 goniometer using with graphite-monochromated Mo Kα radiation (λ=0.71073 §). The sample was cooled to 138° K. The intensity data, which nominally covered one and a half hemispheres of reciprocal space, were measured as a series of φ oscillation frames each of 0.4° for 30 sec/frame. The detector was operated in 512×512 mode and was positioned 5.00 cm from the sample. Coverage of unique data was 98.9% complete to 25.00 degrees in θ. Cell parameters were determined from a non-linear least squares fit of 3054 peaks in the range 3.0<θ<25.0°. The first 50 frames were repeated at the end of data collection and yielded a total of 140 peaks showing a variation of −0.15% during the data collection. A total of 6364 data were measured in the range 1.96<θ<28.20°. The data were corrected for absorption by the empirical method, G. M. Sheldrick (1996), SADABS, Program for Empirical Absorption Correction of Area Detector Data, University of Göttingen, Germany, giving minimum and maximum transmission factors of 0.744 and 0.970. The data were merged to form a set of 4597 independent data with R(int)=0.0320.

The Monoclinic space group C2 was determined by systematic absences and statistical tests and verified by subsequent refinement. The structure was solved by direct methods and refined by full-matrix least-squares methods on F², (a) G. M. Sheldrick (1994), SHELXTL Version 5 Reference Manual. Bruker-AXS, 6300 Enterprise Dr., Madison, Wis. 53719-1173, USA; (b) International Tables for Crystallography, Vol C, Tables 6.1.1.4, 4.2.6.8, and 4.2.4.2, Kluwer: Boston (1995). Hydrogen atom positions were initially determined by geometry and refined by a riding model. Non-hydrogen atoms were refined with anisotropic displacement parameters. A total of 281 parameters were refined against 3 restraints and 4597 data to give wR(F²)=0.1311 and S=0.938 for weights of w=1/[σ² (F²)+(0.0734 p)²], where P=[F_(o) ²+2F_(c) ²]/3. The final R(F) was 0.0522 for the 3133 observed, [F>4σ(F)], data. The largest shift/s.u. was 0.001 in the final refinement cycle. The final difference map had maxima and minima of 0.317 and −0.295 e/§³, respectively. The absolute structure was determined by refinement of the Flack parameter, H. D. Flack, Acta Cryst. A39, 876-881 (1983). The polar axis restraints were taken from Flack and Schwarzenbach, H. D. Flack and D. Schwarzenbach, Acta Cryst. A44, 499-506 (1988).

The displacement ellipsoids were drawn at the 50% probability level. Methyl group C(2) was disordered and modeled in two orientations with occupancies of 0.661(9) for the unprimed atom and 0.339(9) for the primed atom. Restraints were applied to the positional parameters of these atoms.

The three dimensional structure of 1α-hydroxyvitamin D₂ as defined by the following physical data and atomic positional parameters described and calculated herein is illustrated in FIG. 5.

TABLE 1 Crystal data and structure refinement for 98247. Identification code 98247 Empirical formula C28 H45 O2 Formula weight 413.64 Crystal system Monoclinic Space group C2 Unit cell dimensions a = 23.952(4) Å α = 90° b = 6.8121(9) Å β = 119.579(2)° c = 17.994(2) Å γ = 90° Volume 2553.3(6) Å³ Z 4 Density (calculated) 1.076 Mg/m³ Wavelength 0.71073 Å Temperature 138(2)K F(000) 916 Absorption coefficient 0.065 mm⁻¹ Absorption correction Empirical Max. and min. transmission 0.970 and 0.744 Theta range for data collection 1.96 to 28.20°. Reflections collected 6364 Independent reflections 4597 [R(int) = 0.0320] Data/restraints/parameters 4597/3/281 wR(F²all data) wR2 = 0.1311 R(F obsd data) R1 = 0.0522 Goodness-of-fit on F² 0.938 Observed data [I > 2σ(I)] 3.133 Absolute structure parameter 1.2(19) Largest and mean shift/s.u. 0.001 and 0.000 Largest diff. peak and hole 0.317 and −0.295 e/Å³

TABLE 2 Atomic coordinates and equivalent isotropic displacement parameters for 98247. U(eq) is defined as one third of the trace of the orthogonalized U_(ij) tensor. x y z U(eq) O(1) 0.54852(13) 0.4364(3) 0.97881(18) 0.0713(8) O(2) 0.56423(10) 1.0480(3) 1.00431(14) 0.0465(5) C(1) 0.57345(13) 0.6257(4) 0.98084(19) 0.0355(6) C(2) 0.59343(13) 0.7273(4) 1.06595(17) 0.0350(7) C(3) 0.61677(13) 0.9353(4) 1.06655(17) 0.0349(6) C(4) 0.67265(13) 0.9337(4) 1.04799(16) 0.0327(6) C(5) 0.65759(12) 0.8180(4) 0.96806(16) 0.0281(6) C(6) 0.66835(12) 0.8929(4) 0.90804(16) 0.0278(6) C(7) 0.64998(12) 0.8085(4) 0.82523(16) 0.0290(6) C(8) 0.66086(12) 0.8835(4) 0.76415(16) 0.0282(6) C(9) 0.69983(14) 1.0672(4) 0.77547(18) 0.0364(7) C(10) 0.62870(12) 0.6216(4) 0.96228(17) 0.0303(6) C(11) 0.74852(13) 1.0392(4) 0.74434(17) 0.0343(6) C(12) 0.71843(13) 0.9532(4) 0.65447(16) 0.0303(6) C(13) 0.68376(11) 0.7601(4) 0.64784(15) 0.0266(6) C(14) 0.63338(12) 0.8017(4) 0.67657(16) 0.0301(6) C(15) 0.59228(13) 0.6150(5) 0.65149(17) 0.0385(7) C(16) 0.59231(14) 0.5432(5) 0.57008(18) 0.0461(8) C(17) 0.63841(12) 0.6810(4) 0.55701(16) 0.0307(6) C(18) 0.73112(12) 0.6019(4) 0.70394(17) 0.0343(6) C(19) 0.64831(14) 0.4570(4) 0.94245(18) 0.0391(7) C(20) 0.66588(13) 0.5767(4) 0.50566(17) 0.0346(6) C(21) 0.71140(13) 0.7081(4) 0.49045(19) 0.0408(7) C(22) 0.61101(13) 0.5120(4) 0.42223(17) 0.0366(7) C(23) 0.59363(14) 0.3296(5) 0.39507(19) 0.0430(7) C(24) 0.53471(15) 0.2714(5) 0.3132(2) 0.0515(9) C(25) 0.5488(2) 0.1850(5) 0.2454(3) 0.0788(13) C(26) 0.5847(2) 0.3260(6) 0.2199(2) 0.0744(12) C(27) 0.5811(3) −0.0047(7) 0.2677(3) 0.055(2) C(27′) 0.5146(6) −0.0003(16) 0.2045(8) 0.110(7) C(28) 0.49294(18) 0.1351(6) 0.3345(3) 0.0771(12)

TABLE 3 Bond lengths [Å] and angles [°] for 98247. O(1)-C(1) 1.414(3) C(10)-C(1)-C(2) 110.7(2) O(2)-C(3) 1.427(3) C(3)-C(2)-C(1) 111.2(2) C(1)-C(10) 1.517(3) O(2)-C(3)-C(2) 108.5(2) C(1)-C(2) 1.526(4) O(2)-C(3)-C(4) 111.1(2) C(2)-C(3) 1.522(4) C(2)-C(3)-C(4) 110.4(2) C(3)-C(4) 1.531(4) C(5)-C(4)-C(3) 112.7(2) C(4)-C(5) 1.519(4) C(6)-C(5)-C(10) 124.0(2) C(5)-C(6) 1.330(3) C(6)-C(5)-C(4) 121.3(3) C(5)-C(10) 1.486(4) C(10)-C(5)-C(4) 114.7(2) C(6)-C(7) 1.448(3) C(5)-C(6)-C(7) 126.9(2) C(7)-C(8) 1.349(3) C(8)-C(7)-C(6) 127.3(2) C(8)-C(14) 1.484(3) C(7)-C(8)-C(14) 124.1(2) C(8)-C(9) 1.514(4) C(7)-C(8)-C(9) 123.9(2) C(9)-C(11) 1.535(4) C(14)-C(8)-C(9) 111.9(2) C(10)-C(19) 1.331(4) C(8)-C(9)-C(11) 112.2(2) C(11)-C(12) 1.525(3) C(19)-C(10)-C(5) 124.3(2) C(12)-C(13) 1.529(3) C(19)-C(10)-C(1) 122.4(2) C(13)-C(18) 1.529(3) C(5)-C(10)-C(1) 113.3(2) C(13)-C(17) 1.546(3) C(12)-C(11)-C(9) 112.8(2) C(13)-C(14) 1.556(3) C(11)-C(12)-C(13) 111.6(2) C(14)-C(15) 1.534(4) C(12)-C(13)-C(18) 111.0(2) C(15)-C(16) 1.545(4) C(12)-C(13)-C(17) 117.0(2) C(16)-C(17) 1.553(4) C(18)-C(13)-C(17) 111.0(2) C(17)-C(20) 1.546(3) C(12)-C(13)-C(14) 107.3(2) C(20)-C(22) 1.493(4) C(18)-C(13)-C(14) 110.7(2) C(20)-C(21) 1.536(4) C(17)-C(13)-C(14) 99.18(18) C(22)-C(23) 1.325(4) C(8)-C(14)-C(15) 120.5(2) C(23)-C(24) 1.505(4) C(8)-C(14)-C(13) 114.3(2) C(24)-C(25) 1.534(4) C(15)-C(14)-C(13) 104.0(2) C(24)-C(28) 1.545(4) C(14)-C(15)-C(16) 104.0(2) C(25)-C(27) 1.457(4) C(15)-C(16)-C(17) 106.6(2) C(25)-C(27′) 1.486(6) C(13)-C(17)-C(20) 120.5(2) C(25)-C(26) 1.505(5) C(13)-C(17)-C(16) 103.4(2) O(1)-C(1)-C(10) 112.4(2) C(20)-C(17)-C(16) 110.9(2) O(1)-C(1)-C(2) 111.0(2) C(22)-C(20)-C(21) 110.1(2) C(22)-C(20)-C(17) 108.3(2) C(27)-C(25)-C(27) 58.1(6) C(21)-C(20)-C(17) 112.8(2) C(27)-C(25)-C(26) 110.1(4) C(23)-C(22)-C(20) 127.5(3) C(27′)-C(25)-C(26) 130.4(6) C(22)-C(23)-C(24) 125.4(3) C(27)-C(25)-C(24) 114.6(4) C(23)-C(24)-C(25) 114.3(3) C(27′)-C(25)-C(24) 115.9(5) C(23)-C(24)-C(28) 108.9(3) C(26)-C(25)-C(24) 112.4(3) C(25)-C(24)-C(28) 112.9(3)

TABLE 4 Anisotropic displacement parameters (Å² × 10³) for 98247. The anisotropic displacement factor exponent takes the form: −2 π²[h²a*²U₁₁ +... +2hka*b*U₁₂] U₁₁ U₂₂ U₃₃ U₂₃ U₁₃ U₁₂ O(1) 123(2) 19(1) 138(2) −9(1) 115(2) −12(1) O(2) 58(1) 34(1) 68(1) 17(1) 46(1) 21(1) C(1) 49(2) 17(1) 61(2) 4(1) 42(2) 2(1) C(2) 42(2) 30(2) 47(2) 12(1) 33(1) 9(1) C(3) 45(2) 32(2) 39(2) 6(1) 28(1) 7(1) C(4) 39(2) 31(2) 36(1) 5(1) 24(1) 4(1) C(5) 29(1) 24(1) 34(1) 4(1) 18(1) 6(1) C(6) 25(1) 25(1) 35(1) 1(1) 17(1) 0(1) C(7) 28(1) 26(1) 35(1) 2(1) 16(1) 0(1) C(8) 27(1) 25(1) 34(1) 2(1) 16(1) 2(1) C(9) 45(2) 30(2) 40(2) −1(1) 25(1) −8(1) C(10) 38(2) 21(1) 41(2) 9(1) 27(1) 8(1) C(11) 41(2) 28(2) 40(2) 0(1) 25(1) −9(1) C(12) 33(1) 29(1) 33(1) 5(1) 20(1) 2(1) C(13) 23(1) 28(2) 32(1) 5(1) 16(1) 2(1) C(14) 26(1) 33(2) 33(1) 3(1) 16(1) 1(1) C(15) 30(1) 50(2) 42(2) −13(1) 23(1) −12(1) C(16) 36(2) 62(2) 46(2) −15(2) 25(1) −13(2) C(17) 25(1) 38(2) 31(1) −2(1) 15(1) 1(1) C(18) 35(2) 28(2) 43(2) 5(1) 21(1) 2(1) C(19) 45(2) 37(2) 44(2) 10(1) 29(2) 9(1) C(20) 37(2) 33(2) 40(2) −3(1) 23(1) −1(1) C(21) 44(2) 41(2) 51(2) −10(1) 34(2) −7(1) C(22) 43(2) 32(2) 42(2) −1(1) 26(1) −1(1) C(23) 46(2) 42(2) 53(2) −5(2) 34(2) −4(2) C(24) 55(2) 52(2) 65(2) −31(2) 43(2) −24(2) C(25) 103(3) 67(3) 105(3) −42(2) 81(3) −38(2) C(26) 100(3) 76(3) 82(3) −27(2) 71(2) −28(2) C(27) 84(4) 43(3) 61(4) 1(2) 54(3) 6(3) C(27′) 87(11) 166(17) 81(10) −77(10) 45(9) −31(10) C(28) 77(3) 79(3) 106(3) −40(2) 70(2) −40(2)

TABLE 5 Hydrogen coordinates and isotropic displacement parameters for 98247. x y z U(eq) H(1A) 0.5187 0.4364 0.9914 0.107 H(1B) 0.5682 0.3243 1.0063 0.107 H(2A) 0.5737 1.1693 0.9770 0.070 H(2B) 0.5319 1.0320 1.0104 0.070 H(1) 0.5382 0.7054 0.9350 0.043 H(2A) 0.5564 0.7314 1.0761 0.042 H(2B) 0.6281 0.6510 1.1128 0.042 H(3) 0.6318 0.9944 1.1242 0.042 H(4A) 0.6833 1.0705 1.0412 0.039 H(4B) 0.7108 0.8758 1.0974 0.039 H(6) 0.6904 1.0149 0.9208 0.033 H(7) 0.6278 0.6868 0.8125 0.035 H(9A) 0.7230 1.1039 0.8367 0.044 H(9B) 0.6704 1.1762 0.7433 0.044 H(11A) 0.7681 1.1676 0.7450 0.041 H(11B) 0.7832 0.9509 0.7844 0.041 H(12A) 0.7524 0.9301 0.6393 0.036 H(12B) 0.6875 1.0486 0.6132 0.036 H(14) 0.6048 0.9068 0.6375 0.036 H(15A) 0.5481 0.6445 0.6395 0.046 H(15B) 0.6114 0.5154 0.6974 0.046 H(16A) 0.5485 0.5500 0.5202 0.055 H(16B) 0.6075 0.4056 0.5772 0.055 H(17) 0.6123 0.7945 0.5218 0.037 H(18A) 0.7505 0.6404 0.7641 0.051 H(18B) 0.7084 0.4770 0.6951 0.051 H(18C) 0.7648 0.5868 0.6885 0.051 H(19A) 0.6649 0.3664 0.9910 0.059 H(19B) 0.6121 0.3958 0.8927 0.059 H(19C) 0.6823 0.4877 0.9290 0.059 H(20) 0.6901 0.4578 0.5382 0.042 H(21A) 0.7217 0.6437 0.4500 0.061 H(21B) 0.6905 0.8343 0.4669 0.061 H(21C) 0.7510 0.7300 0.5447 0.061 H(22) 0.5856 0.6132 0.3842 0.044 H(23) 0.6206 0.2269 0.4302 0.052 H(24) 0.5093 0.3940 0.2883 0.062 H(25) 0.5062 0.1628 0.1932 0.095 H(25′) 0.5869 0.1178 0.2864 0.095 H(26A) 0.5891 0.2695 0.1730 0.112 H(26B) 0.5611 0.4500 0.2013 0.112 H(26C) 0.6274 0.3502 0.2689 0.112 H(27A) 0.5881 −0.0496 0.2212 0.083 H(27B) 0.6225 0.0083 0.3202 0.083 H(27C) 0.5544 −0.1002 0.2767 0.083 H(27D) 0.5271 −0.0434 0.1627 0.164 H(27E) 0.5261 −0.1018 0.2482 0.164 H(27F) 0.4682 0.0222 0.1754 0.164 H(28A) 0.4777 0.2077 0.3681 0.116 H(28B) 0.4561 0.0889 0.2813 0.116 H(28C) 0.5186 0.0224 0.3676 0.116

TABLE 6 Torsion angles [°] for 98247. O(1)-C(1)-C(2)-C(3) −176.8(2) C(12)-C(13)-C(14)-C(8) −57.5(3) C(10)-C(1)-C(2)-C(3) 57.6(3) C(18)-C(13)-C(14)-C(8) 63.7(3) C(1)-C(2)-C(3)-O(2) 64.8(3) C(17)-C(13)-C(14)-C(8) −179.7(2) C(1)-C(2)-C(3)-C(4) −57.2(3) C(12)-C(13)-C(14)-C(15) 169.2(2) O(2)-C(3)-C(4)-C(5) −69.0(3) C(18)-C(13)-C(14)-C(15) −69.6(2) C(2)-C(3)-C(4)-C(5) 51.4(3) C(17)-C(13)-C(14)-C(15) 47.0(2) C(3)-C(4)-C(5)-C(6) 130.8(3) C(8)-C(14)-C(15)-C(16) −162.1(2) C(3)-C(4)-C(5)-C(10) −47.5(3) C(13)-C(14)-C(15)-C(16) −32.4(3) C(10)-C(5)-C(6)-C(7) 5.3(4) C(14)-C(15)-C(16)-C(17) 5.0(3) C(4)-C(5)-C(6)-C(7) −172.8(2) C(12)-C(13)-C(17)-C(20) 77.7(3) C(5)-C(6)-C(7)-C(8) −179.8(3) C(18)-C(13)-C(17)-C(20) −51.0(3) C(6)-C(7)-C(8)-C(14) −171.8(2) C(14)-C(13)-C(17)-C(20) −167.4(2) C(6)-C(7)-C(8)-C(9) 4.9(4) C(12)-C(13)-C(17)-C(16) −157.8(2) C(7)-C(8)-C(9)-C(11) 133.3(3) C(18)-C(13)-C(17)-C(16) 73.5(3) C(14)-C(8)-C(9)-C(11) −49.6(3) C(14)-C(13)-C(17)-C(16) −42.9(3) C(6)-C(5)-C(10)-C(19) 49.5(4) C(15)-C(16)-C(17)-C(13) 24.2(3) C(4)-C(5)-C(10)-C(19) −132.3(3) C(15)-C(16)-C(17)-C(20) 154.8(2) C(6)-C(5)-C(10)-C(1) −130.1(3) C(13)-C(17)-C(20)-C(22) 178.3(2) C(4)-C(5)-C(10)-C(1) 48.1(3) C(16)-C(17)-C(20)-C(22) 57.5(3) O(1)-C(1)-C(10)-C(19) 3.0(4) C(13)-C(17)-C(20)-C(21) −59.6(3) C(2)-C(1)-C(10)-C(19) 127.8(3) C(16)-C(17)-C(20)-C(21) 179.5(2) O(1)-C(1)-C(10)-C(5) −177.3(2) C(21)-C(20)-C(22)-C(23) 121.8(3) C(2)-C(1)-C(10)-C(5) −52.5(3) C(17)-C(20)-C(22)-C(23) −114.4(3) C(8)-C(9)-C(11)-C(12) 50.4(3) C(20)-C(22)-C(23)-C(24) 174.8(3) C(9)-C(11)-C(12)-C(13) −55.2(3) C(22)-C(23)-C(24)-C(25) 110.7(4) C(11)-C(12)-C(13)-C(18) −64.9(3) C(22)-C(23)-C(24)-C(28) −121.9(3) C(11)-C(12)-C(13)-C(17) 166.5(2) C(23)-C(24)-C(25)-C(27) 67.2(4) C(11)-C(12)-C(13)-C(14) 56.2(3) C(28)-C(24)-C(25)-C(27) −58.1(5) C(7)-C(8)-C(14)-C(15) −3.1(4) C(23)-C(24)-C(25)-C(27′) 132.1(8) C(9)-C(8)-C(14)-C(15) 179.9(2) C(28)-C(24)-C(25)-C(27′) 6.8(8) C(7)-C(8)-C(14)-C(13) −128.0(3) C(23)-C(24)-C(25)-C(26) −59.5(5) C(9)-C(8)-C(14)-C(13) 54.9(3) C(28)-C(24)-C(25)-C(26) 175.2(4)

TABLE 7 Hydrogen bonds for 98247[Å and °]. D-H...A d(D-H) d(H...A) d(D...A) <(DHA) O(1)-H(1A)...O(1)#1 0.85 1.93 2.781(4) 179.4 O(1)-H(1B)...O(2)#2 0.91 1.88 2.679(3) 145.5 O(2)-H(2A)...O(1)#3 1.04 1.92 2.679(3) 126.8 O(2)-H(2B)...O(2)#1 0.84 2.19 3.004(4) 163.7 Symmetry transformations used to generate equivalent atoms: #1 −x + 1, y, −z + 2 #2x, y − 1, z #3 x, y + 1, z 

What is claimed is:
 1. A method of purifying 1α-hydroxyvitamin D₂, comprising the steps of: (a) boiling a solvent consisting essentially of ethyl formate under inert atmosphere; (b) dissolving a product containing 1α-hydroxyvitamin D₂ to be purified in said solvent; (c) cooling said solvent and dissolved product below ambient temperature for a sufficient amount of time to form a precipitate of 1α-hydroxyvitamin D₂ crystals; and (d) recovering the 1α-hydroxyvitamin D₂ crystals having a molecular packing arrangement defined by space group C2 and unit cell dimensions a=23.9A°, b=6.8A°, c=17.9A°, α=90°, β=119.5° and γ=90°.
 2. The method of claim 1 wherein said solvent and dissolved product is allowed to cool to ambient temperature prior to cooling below ambient temperature.
 3. The method of claim 1 wherein said inert atmosphere is an argon atmosphere.
 4. The method of claim 1 wherein said solvent and dissolved product is cooled to between about 35° F. to about 45° F.
 5. The method of claim 1 wherein the step of recovering comprises filtering.
 6. The method of claim 1 wherein steps (a) through (d) are repeated using the recovered crystals from step (d) as the product in step (b).
 7. A method of purifying 1α-hydroxyvitamin D₂, comprising the steps of: (a) boiling a solvent consisting essentially of ethyl acetate under inert atmosphere; (b) dissolving a product containing 1α-hydroxyvitamin D₂ to be purified in said solvent; (c) adding petroleum ether to said solvent after dissolving said product in said solvent; (d) cooling said solvent and dissolved product below ambient temperature for a sufficient amount of time to form a precipitate of 1α-hydroxyvitamin D₂ crystals; and (e) recovering the 1α-hydroxyvitamin D₂ crystals having a molecular packing arrangement defined by space group C2 and unit cell dimensions a=23.9A°, b=6.8A°, c=17.9A°, α=90°, β=119.5° and γ=90°.
 8. The method of claim 7 wherein said solvent and dissolved product is allowed to cool to ambient temperature prior to cooling below ambient temperature.
 9. The method of claim 7 wherein said inert atmosphere is an argon atmosphere.
 10. The method of claim 7 wherein said solvent and dissolved product is cooled to between about 35° F. to about 45° F.
 11. The method of claim 7 wherein the step of recovering comprises filtering.
 12. The method of claim 7 herein steps (a) through (e) are repeated using the recovered crystals from step (e) as the product in step (b).
 13. A method of purifying 1α-hydroxyvitamin D₂, comprising the steps of: (a) boiling a solvent consisting essentially of 2-propanol-hexane mixture under inert atmosphere; (b) dissolving a product containing 1α-hydroxyvitamin D₂ to be purified in said solvent; (c) adding petroleum ether to said solvent after dissolving said product in said solvent; (d) cooling said solvent and dissolved product below ambient temperature for a sufficient amount of time to form a precipitate of 1α-hydroxyvitamin D₂ crystals; and (e) recovering the 1α-hydroxyvitamin D₂ crystals having a molecular packing arrangement defined by space group C2 and unit cell dimensions a=23.9A°, b=6.8A°, c=17.9A°, α=90°, β=119.5° and γ=90°.
 14. The method of claim 13 wherein said solvent and dissolved product is allowed to cool to ambient temperature prior to cooling below ambient temperature.
 15. The method of claim 13 wherein said inert atmosphere is an argon atmosphere.
 16. The method of claim 13 wherein said solvent and dissolved product is cooled to between about 35° F. to about 45° F.
 17. The method of claim 13 wherein the step of recovering comprises filtering.
 18. The method of claim 13 wherein steps (a) through (e) are repeated using the recovered crystals from step (e) as the product in step (b). 